Tuning single target detection algorithms

Simmonds and MacLennan discuss single targets in their chapters Observation and Measurement of Fish and Target Strength of Fish. Single targets are single-fish echoes that come from isolated fish and may be used for fish counting and biomass estimation. A technique for identifying single targets uses aspects and characteristics of the shape of the return pulse. Echoview single target detection settings allow you to specify acceptable values for some of these aspects. Some understanding of the pulse shape and of the characteristics of your data can guide you in choosing reasonable single target detection settings.

Finding high quality single targets is a task that balances:

settings that are not so tight that you fail to detect true single targets
settings that are not so open that you detect too many false single targets

Another consideration is how single targets affect fish track detection. Fish track detection benefits from slightly more open single target settings. You could reconcile single target detection needs with fish track detection needs by using two single target variables - one for each purpose.

Specific setting descriptions and information:

Single-fish echo parameters:

Single-fish echo	Description
Threshold	The (TS) threshold determines the part of the received signal that is considered to be an echo from one or more targets (Simmonds and MacLennan, 5.3.1). Received signal below the threshold is ignored. "The threshold ... should be small enough to observe the lower range of the TS distribution ..." (Parker-Stetter et al: Section 9.2 Single-echo detection).
Echo length	Single objects will not have very small or very long normalized pulse lengths. A true single target will have a normalized pulse length around 1 ± 0.5. "... One criterion is to accept only those echoes whose duration is within some interval around τ... " (Simmonds and MacLennan, 5.3.1)
Echo shape	Single objects have a typical echo shape (Simmonds and MacLennan, Fig 5.10). Multiple objects and noise alter the shape of a single echo.
Phase deviation	Split beam data contain angular position and phase information that tell you where targets are in the beam.
Angular position	Split beam data contain angular position and phase information that tell you where targets are in the beam.
Beam compensation	A transducer has an associated beam pattern. Beam patterns can be modeled and may be used to estimate what the TS value would be if the target was central in the beam. Models have limitations and a range for valid application. The correction of the target strength uses the known angular position in the beam and the known beam strength variation with angle. The analysis variables Beam_volume_sum and Wedge_volume_sampled are affected by the value for Maximum beam compensation. Refer to Beam angle parameters for further information.

It can be useful to relax the criteria for single target detection in order to examine the field of possible targets when tuning the detection. However, the possible single target detections may contain a mix of true and false single target detections.

Other useful considerations:

Noise, background object and signal removal in Echoview
Estimate the SNR and the range limit for single target detection (Parker-Setter et al, Standard operating procedures for fisheries acoustic surveys in the Great Lakes: Section 3.6.1).
Account for aspects that may affect the single target detection probability. Aspects like organism orientation and organism behavior (Parker-Setter et al, Section 3.6 Detection Probability).
Employ image processing algorithms or filters provided by Echoview operators. These may improve the SNR of single targets that fit well with fish tracks.
Cell statistics operators output values that may be used to calculate density indices to find the best areas for single-fish counting. These are where fish are randomly distributed at a low average density (Simmonds and MacLennan, Observation and Measurement of Fish: Single-target echoes).
Set Exclusion lines on the Single Target Detection page of the Variable Properties dialog box.
Use settings on the Filter Single Targets page to filter single target detections (and remove spike noise when present in your data).
The Graph ping feature is useful in examining TS data pulse shape.

Tuning single target detection algorithms

See also: